1. Field of the Invention
This invention relates generally to an improved engine braking system of the compression relief type. More particularly, the invention relates to a turbocharged internal combustion engine fitted with a compression relief type of engine brake wherein the turbocharger comprises a double entry turbine together with a diverter valve and the method of operating such apparatus to provide improved engine braking and improved engine performance.
2. The Prior Art
The problem of providing adequate and reliable braking for vehicles, particularly large tractor-trailer vehicles is well known. When such vehicles are operating at normal highway speed they possess a very large momentum, and this may be increased substantially when the vehicle is required to negotiate a long decline. While the normal drum or disc type wheel brakes are capable of absorbing a large amount of energy over a short period of time, the absorbed energy is transformed into heat which rapidly raises the temperature of the braking mechanism to a level which may render ineffective the friction surfaces and other parts of the mechanism. Repeated use of the wheel brakes under these conditions is thus impracticable and resort has been made to auxiliary braking devices.
Such devices include hydraulic or electrodynamic braking systems whereby the kinetic energy of the vehicle is transformed by fluid friction or magnetic eddy currents into heat which may be dissipated through heat exchangers. Other auxiliary braking systems include exhaust brakes which inhibit the flow of exhaust gases through the exhaust system and compression relief mechanisms wherein the energy required to compress the intake air during the compression stroke of the engine is dissipated by exhausting the compressed air through the exhaust system. With respect to both the exhaust brake and the engine compression relief brake, a portion of the kinetic energy of the vehicle is dissipated as heat through the engine cooling system while another portion of the kinetic energy is dissipated through the engine exhaust system.
One principal advantage of the engine compression relief brake and the exhaust brake over the hydraulic and electrodynamic brakes is that both of the latter brakes require dynamos or turbine equipment which may be bulky and expensive in comparison with the mechanism required for the usual exhaust brake or engine compression relief brake. A typical engine compression relief brake is shown in the Cummins U.S. Pat. No. 3,220,392 while an exhaust brake is disclosed in Benson U.S. Pat. No. 4,054,156.
The concept of supercharging an engine in order to increase the mass flow of air has long been established in the art for both land vehicles and aircraft. Such supercharging is frequently accomplished by means of a turbocharger incorporating an exhaust gas turbine coupled to a centrifugal compressor. While various types of turbines may be employed, a radial flow turbine is a common choice. In such a turbine it may be desirable to use a divided volute so that the exhaust gas flow may be directed to either or both portions of the volute, as desired. Such a construction is shown in Webster U.S. Pat. No. 3,557,549, Neff U.S. Pat. No. 3,614,259, Clarke U.S. Pat. No. 2,247,151, Cowland U.S. Pat. No. 2,838,907, Connor U.S. Pat. No. 3,270,495 and Nancarrow U.S. Pat. No. 3,423,926. Where a divided volute turbine is used, it is necessary to provide means to direct the gas flow into either or both portions of the volute. The patents cited immediately above disclose various forms of diverter mechanisms for this purpose and other diverter valves are shown in Woollenweber, Jr. U.S. Pat. No. 4,008,572, Navarro U.S. Pat. No. 3,559,397, Kofink U.S. Pat. No. 3,137,477, Nancarrow U.S. Pat. No. 3,313,518 and Morgulis U.S. Pat. No. 3,975,911.
As disclosed in various of the patents cited above, the volume of exhaust gas available to drive the turbine of the turbocharger varies with the engine speed and engine throttle settings. A decrease in exhaust gas volume results in a decrease in the gas velocity at the turbine nozzles and a reduced turbine efficiency as well as speed. It has been found desirable in order to improve the efficiency of the turbine at low exhaust gas flow rates to divert all of the exhaust gas to a portion of the turbine or to selected inlet nozzles so as to maintain the gas velocity at the inlet nozzle. As shown in the Webster U.S. Pat. No. 3,557,549 and the Neff U.S. Pat. No. 3,614,259, for example, a diverter valve of the flapper type directs the exhaust gas into one or both portions of the divided volute of the turbine. In the Clarke U.S. Pat. No. 2,247,151 sequentially operated diverter valves direct the exhaust gas to various segments of the turbine. Another approach is represented by the Kofink U.S. Pat. No. 3,137,477 which discloses the concept of changing the number of nozzles through which the exhaust gas flows.